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Luxman KMQ and MQ-68c updates

Iain42, hi. Tim de P did the MB-3045 monos. The KMQ and the MQ-68c came after he had departed. The MQ-68c was circa 1981-2. Both KMQ and 68c look to have exactly the same chassis and layout with minor differences. The power switch is on the front of the KMQ and on the back of the 68c.

Tube lineup looks like: 6267/EF86 first, 6240G phase splitter/driver, and 50C-A10 outputs. 6FQ7/6CG7 is a direct replacement for the 6240G. There is no replacement for the 50C-A10. It has a 50 Volt filament. There was a 6 V filament version of this Japanese tube (6C-A10) but for whatever reason Lux chose not to use it. Go figure! It has a 12 pin Noval base (compactron in GE lingo) with exhaust tip out the bottom of the button base.

To properly restore the MQ-68c, and KMQ for everyday use I see no other course but an outboard box for power supply C and filament transformer for output tubes. Also outboard VR for easy bias set and balance. I'd use 10 turn wire wounds.

I'll wager the electrolytic caps in your KMQ are: a double at 550V, a triple at 550V, and a triple with + common 350V for bias.

The outputs look like the Lux OY series. 6K primary. Good outputs. Only one 8 Ohm secondary though.

Watch out for the cheap trim pots Lux used in this amp (and other amps). There are 2 10K bias set trimmers and 2 5K bias balance trimmers. To bias/balance you must invert the amp and take the bottom plate off.

There are (probably) 2 additional 5K trimmers per channel for AC and DC balance for which you also need: a dummy load for the amp and a distortion analyzer. These trimmers are in the B+ feed for the phase splitter and also in the Cathodes to ground circuit for the same tube. Set these so R to electrodes is the same on both sides - in other words so there is no R imbalance if you do not have a distorion analyzer etc.

You'll probably want to replace the ancient silicon diodes full wave bridge and bias with something better - Fairchild Stealth for example.

Oh, and the sooner you have a proper step down transformer the better. A variac (autoformer) is not good for the sound.





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